1. Field of the Invention
The present invention relates to a drive device and an image forming apparatus that use a grease composition, and the grease composition used for the drive device and the image forming apparatus.
2. Description of the Related Art
Image forming apparatuses employing an electrophotographic process use many drive devices in mechanisms for an image reading process, an image forming process, a transfer process, and a paper conveyance process, for example. Conventional image forming apparatuses are large-sized and handled as precision apparatuses. The image forming apparatuses are, thus, often installed in places apart from people who perform office operations in offices such as dedicated rooms provide for the apparatuses. Sounds generated during the process for forming images by such image forming apparatuses, thus, are not a serious problem. In fact, the start of forming an image is checked by the generated sound and the end of forming the image is recognized by the stop of the sound.
With the progress of downsizing of the image forming apparatuses in recent years, cases have been increased in which the image forming apparatuses are installed in places just near users such as on the user's desks or side desks. A plurality of users often access an image forming apparatus through a local area network (LAN) to instruct the image forming apparatus to form an image using user's personal computers. As a result, the operating rate of the image forming apparatus tends to be increased. Sounds generated by the image forming apparatuses, which have not been a serious problem, can be often hard to bear for the users near the place in which the image forming apparatuses are installed. In addition, offices are quiet these days. Sounds generated from the image forming apparatuses, thus, become more noticeable.
Most of the generation sources of sounds from the image forming apparatuses are the drive devices described above. The drive devices drive objects by transmitting kinetic energy from the driving sources such as motors to the objects through gears and belts, for example. The image forming apparatuses each include many drive devices. In particular, the gears are very important parts for the transmission of the kinetic energy of the driving sources. Such a drive device is usually provided with many gears. A sound is generated from the drive device due to rubbing between tooth flanks of engaged gears, rubbing between a slide bearing and a shaft to which a gear is fixed, and rubbing between a shaft fixed non-rotatably and the gear rotating on the shaft, for example.
As examples the image forming apparatuses that reduce a noise generated by rubbing between the tooth flanks of the gears, image forming apparatuses in Japanese Patent Application Laid-open No. 2010-083658 and Japanese Patent Application Laid-open No. 2003-312868 have been developed that reduce a noise by a grease composition applied on the tooth flanks of the gears. In Japanese Patent Application Laid-open No. 2001-228660, a drive device is disclosed that uses gears having grooves to hold a grease composition for preventing the grease composition from coming off from the tooth flanks. As examples such as the image forming apparatuses and the drive devices described above, conventionally, it has been common practice to apply a grease composition used for reducing a noise mainly on gears. It is considered that a slightly hard grease composition containing a solid lubricant such as polytetrafluoroethylene, molybdenum disulfide, graphite, or melamine cyanurate has a high effect of reducing a noise. Such a grease composition prevents a hitting sound from being generated by a direct contact between the gears, and reduces friction and wear between the gears, thereby making it possible to maintain the smooth rotation of the gears. The reason why a slightly hard grease composition, specifically, a grease composition having a low consistency, is used is that an excessively soft grease composition may come off from the tooth flanks by a centrifugal force during the operation of the drive device. Conventionally, it has been considered that a noise from the drive device is mainly generated from the tooth flanks of gears and a noise generated between a slide bearing and a shaft that passes through the slide bearing (hereinafter the shaft is also described as a member that fixes a gear) is not considerable.
The present inventors have found that it is very important to prevent a noise generated between the slide bearing and the shaft passing through the slide bearing as a result of research for more effectively reducing a noise generated from the drive device. The shaft passing through the slide bearing is often directly connected to a motor serving as a driving source, or disposed at a position at which an object is lastly driven. The shaft passing through the slide bearing rotates at a high speed and torque applied to the shaft, thus, fluctuates relatively largely. It has been found that the slide bearing and the shaft passing through the slide bearing largely influence a noise generated from the whole of the drive device although the noise generated by the sliding movement therebetween is relatively small. A noise generated between the slide bearing and the shaft passing through the slide bearing is caused by the shaft not rotating smoothly. An adverse effect due to the shaft not rotating smoothly is propagated to other gears, thereby causing the generation of a large noise as the whole of the drive device.
The shaft that cannot smoothly rotate in the slide bearing shortens the operating life of the slide bearing. Specifically, the slide bearing is usually often made of a plastic material because the plastic material can achieve weight reduction, easy maintenance of parts, and cost reduction, for example. When the shaft passing through the slide bearing (member that fixes a gear) is made of a metallic material and rotates unstably while being in direct contact with the slide bearing, the shaft readily causes the slide bearing to be worn or damaged. With increasing degree of wear or damage of the slide bearing, the shaft rotates further unstably, thereby causing a large noise. As a result, the slide bearing needs to be replaced at an early stage before the end of its operating life.
The present inventors have conducted experiments to reduce the wear of the slide bearing by providing grease compositions between the slide bearing and the shaft passing through the slide bearing. It has been found that most of the grease compositions containing a solid lubricant provide almost no effect on the rubbing between the slide bearing and the shaft passing through the slide bearing, and in fact some of the grease compositions often worsen the noise. Most of the grease compositions that worsen the noise reduce the noise when they are applied on the tooth flanks of the gears. The reason why such grease compositions often worsen the noise is that a solid lubricant in a large size is caught to the slide bearing or the shaft due to a small clearance between the slide bearing and the shaft passing through the slide bearing, thereby hindering the rotation of the shaft.
The slide bearing made of a plastic material is inferior to that made of a metallic material (including alloys) in dimensional accuracy after molding. It is, thus, difficult to reduce a dimensional variation of the clearance between the slide bearing and the shaft passing through the slide bearing. The solid lubricant having a relatively large size contained in the grease composition is, thus, readily caught to the slide bearing or the shaft, thereby hindering the rotation of the shaft. As a result, a noise is generated.
An object of the present invention is to provide a drive device, an image forming apparatus, and a grease composition that can prevent the occurrence of a noise while maintaining a smooth rotation of a shaft passing through a slide bearing.
According to an embodiment, a drive device includes a slide bearing; a shaft that passes through the slide bearing; a gear that is fixed to the shaft; and a grease composition that is held in a clearance between the slide bearing and the shaft. At least one of the slide bearing and the shaft is made of a resin. The clearance is in a range of 10 to 110 μm. The grease composition contains a hydrocarbon base oil and lithium soap serving as a thickener. A weight ratio of the hydrocarbon base oil to the lithium soap is in a range of 94.5:5.5 to 96.0:4.0. A consistency of the grease composition is in a range of 360 to 400.
The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.